Overview

Brief Summary

The Arthrobacter genus is commonly found amongst soil bacteria. Species that are members of this genus are gram positive, obligate aerobes. They are chemoorganotrophs and have an oxidative metabolism (Holt et al., 1994). Arthrobacter species have been particularly useful in the bioremediation of groundwater contaminated with pesticides and herbicides; this is made possible because of their adaptable genomes, which can handle stressful conditions and environments. (Mongodin et al., 2006).

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Comprehensive Description

Description of Arthrobacter

Arthrobacter is a common genus of soil bacteria. All species in this genus are Gram-positive, obligate aerobes. Like many soil bacteria, this genus is metabolically versatile, producing many different enzymes allowing it to grow on a wide range of substrates. The cells can be quite tough and resist desiccation and starvation. Arthrobacter species have been isolated a few times from patients with immunodeficiencies but most strains do not appear to be pathogenic. A distinctive feature of this genus is that the shape of the cells changes during the growth cycle, typically forming rods during exponential growth and cocci in the stationary phase: Type species: ¤ Arthrobacter globiformis (Conn 1928) Conn and Dimmick 1947. CONN HJ and DIMMICK I. 1947 Soil bacteria similar in morphology to Mycobacterium and Corynebacterium. Journal of Bacteriology, 54, 291-303.

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Physical Description

Morphology

A very important feature of the Arthrobacter genus is their morphology. Specifically, Arthrobacter are observed to be rod-like during the exponential growth phase and coccoid during the stationary phase. This change from rod-like to coccoid during the Arthrobacter growth cycle is influenced by a key nutrient called biotin. The age of a particular culture media is also important; rod-like configurations are associated with fresh, “young” media, whereas the coccoid configuration dominates in older media (Mullakhanbhai and Bhat, 1967). When being observed, Arthrobacter species range from yellow to white in their coloration and measure 2 mm in diameter, on average. They are gram-positive and nonsporulating as well.

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Ecology

Habitat

Arthrobacter species were first found in soils in the 19th century and have since established themselves as one of the dominant bacterial genera found there. The Arthrobacter genus has demonstrated that it is resourceful metabolically because of its ability to grow on many different types of substrates. This genus is not only resourceful metabolically, but also resilient to undesirable environmental conditions. Arthrobacter have been found to survive in places such as the deep subsurface, arctic ice, chemically contaminated sites, and radioactive environments; they can handle starvation, radiation, exposure to oxygen radicals and even toxic chemicals. This incredible ability to survive has been attributed to Arthrobacter’s unique morphological cycle, which allows the species to remain in a stable, coccoid configuration at stressful times (Mongodin et al., 2006).

Tests have been performed to determine what Arthrobacter species can utilize as their carbon sources for growth. Of 130 Arthrobacter isolates tested, seventy-seven percent were able to survive and grow off of at least two aromatic substrates (Stevenson, 1967). Arthrobacter with simple nutritional needs can make use of the most aromatic hydrocarbons and these hydrocarbons can even pass as the lone source of carbon for these species. It is also important to know that Arthrobacter species have an optimal growth temperature of between 20 and 30 C. A neutral to slightly alkaline pH is also preferred for growth (Holt et al., 1994).

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Diseases and Parasites

Although not normally considered to be a threat to our health, there have been cases where Arthrobacter species were identified as the causative agent of disease. In particular, Arthrobacter woluwensis was determined to be the cause of infective endocarditis in a 39-year-old patient who had used intravenous drugs for many years. Outside of this case, Arthrobacter species have only been reported five times as the cause of human disease (Bernasconi et al., 2004). Arthrobacter species have also been isolated in the past from patients with compromised immune systems, but pathogenicity was low (Conn and Dimmick, 1947). Another species, Arthrobacter cumminsii, has actually become the most commonly encountered strain, but is difficult to identify and therefore may be underdiagnosed (Funke et al., 1998).

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Life History and Behavior

Behavior

Motility was originally discovered to be associated with the distinct morphogenic cycles of three Arthrobacter species: Arthrobacteratrocyaneus, Arthrobactercitreus, and Arthrobactersimplex. These members of the genus displayed motility beginning at the time of induction to the rod-shaped morphology, yet were later non-motile when in the coccoid morphology (Stanlake and Clark, 1976).

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Evolution and Systematics

Evolution

Characteristics of Arthrobacter species were being described by H.J. Conn as early as 1928, but it was not until 1957 that the Arthrobacter genus was included as part of the Corynebacteriaceae family (Breed et al., 1957). The major distinguishing feature for Arthrobacter species was the way the species could alternate between rods in young cultures and coccoids in older cultures. This reliance on the morphological characteristics inevitably led to confusion over what actually constituted a member of the genus and led to many misclassifications of species. Over time, more characteristics were outlined as being necessary for inclusion as a member of the Arthrobacter genus, and two groups, which all species must be classified under, were developed. These groups are the A. globiformis/A. citreus group and the A. nicotianae group, and they differ in three important areas including their peptidoglycan construction, teichoic acid content, and lipid composition (Jones and Keddie, 2007).

Based off the results of 16s rRNA studies, Arthrobacter species are related to the following coryneform genera: Aureobacterium, Cellulomonas, Curtobacterium and Microbacterium. Arthrobacter species are also related, albeit less closely, to Brevibacterium (Stackebrandt et al., 1980; Stackebrandt and Woese, 1981). They all appear on the actinomycete branch of a phylogenetic tree for gram-positive eubacteria with high GC content. Also, these Arthrobacter species cannot be separated phylogenetically from members of the Micrococcus genus due to similarities. However, both Arthrobacter and Micrococcus are considered their own taxa (Jones and Keddie, 2007).

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Physiology and Cell Biology

Physiology

Arthrobacter species are obligate aerobes with respiratory metabolisms; there are no species in this genus that have fermentative metabolisms (Holt et al., 1994). Nicotine, nucleic acids, herbicides and pesticides can all be used as substrates for the oxidative metabolism of different Arthrobacter species. Two species have been found to supplement their normal metabolisms with an anaerobic metabolism. This is necessary for Arthrobacter globiformis and Arthrobacter nicotianae, two species that encounter environments that are sometimes limited in oxygen. Also, Arthrobacter can transform hexavalent chromium, which is toxic, into trivalent chromium, which is its less toxic form (Megharaj et al., 2003).

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Molecular Biology and Genetics

Molecular Biology

Seventeen Arthrobacter genomes are available via the National Center for Biotechnology Information (NCBI).

Arthrobacter arilaitensis (Monnet et al., 2010)

Size = 3.92 Mb

GC content = 59.3 %

Arthrobacter phenanthrenivorans

Size = 4.54 Mb

GC content = 65.4%

Arthrobacter chlorophenolicus

Size = 4.98 Mb

GC content = 66.0%

Arthrobacter aurescens (Mongodin et al., 2006)

Size = 5.23 Mb

GC content = 62.4 %

Arthrobacter nitroguajacolicus (Parschat et al., 2007)

Size = 5.08 Mb

GC Content = 60.9 %

Arthrobacter globiformis *

Size = 4.95 Mb

GC content = N/A

Arthrobacter sp. FB24

Size = 5.07 Mb

GC content = 65.4 %

Arthrobacter sp. Rue61a

Size = 5.08 Mb

GC content = 62.2 %

Arthrobacter sp. M2012083 *

Size = 4.63 Mb

GC content = 62.0 %

Arthrobacter sp. TB 23 *

Size = 3.54 Mb

GC content = 63.3 %

Arthrobacter sp. 131MFCol6.1 *

Arthrobacter sp. 135MFCol5.1 *

Arthrobacter sp. 161MFSha2.1 *

Arthrobacter sp. 162MFSha1.1 *

Arthrobacter sp. 388 *

Arthrobacter sp. 9MFCol3.1 *

Arthrobacter sp. MA-N2 *

* Genome sequencing in progress.

Arthrobacter sp. FB24 was isolated from soil that had been polluted with chromate, lead and hydrocarbons. This particular strain was interesting to scientists conducting experiments because it had an extremely high tolerance for the chromium. In fact, it was found to be resistant to other heavy metals and may be radiation resistant as well. Further study of this species may help reveal, from a molecular standpoint, why Arthrobacter can survive such taxing conditions (Nakatsu et al., 2005; Joynt et al., 2006).

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Relevance to Humans and Ecosystems

Benefits

Because of their metabolic diversity, Arthrobacter species have been taken advantage of for their ability to biodegrade various types of pollutants in our environment. Species of the genus, like Arthrobacter sp. FB24, are quite resistant to certain heavy metals that are toxic, which is useful. Arthrobacteraurescens strain TC1 can metabolize more than 23 kids of s-triazine compounds, an important fact to consider since these compounds are found in pesticides, resins, dyes, and explosives (Mongodin et al., 2006). Arthrobacter crystallopoietes can reduce hexavalent chromium levels in soil, which may mean that there are potential future applications for it in bioremediation as well (Camargo et al., 2003).

Colonies of Arthrobacter have a greenish metallic center on mineral salts pyridone broth incubated at 20 °C (68 °F). This genus is distinctive because of its unusual habit of "snapping division" in which the outer bacterial cell wall ruptures at a joint (hence its name). Microbiologists refer to the type of cell division in which rods break into cocci as reversion. Under the microscope, these dividing cells appear as chevrons ("V" shapes). Other notable characteristics are that it can use pyridone as its sole carbon source, and that its cocci are resistant to desiccation and starvation.

Arthrobacter chlorophenolicus sp. nov., a species capable of degrading high concentrations of 4-chlorophenol, may also be useful in bioremediation.[2]Arthrobacter sp. strain R1 (American Type Culture Collection strain number 49987) has been shown to grow on a variety of aromatic compounds, including homocyclic compounds, such as hydroxybenzoates, as well as N-heterocycles, including pyridine and picoline.[3]

The enzyme Alu obtained from Arthrobacter luteus is able to cleave Alu sequences in human DNA, causing the kind of gene sequence to be named after the enzyme. Alu are a group of moderately repetitive gene sequences (300bp in length). Alu sequences make up 6-8% of the human genome.[4]